Alstom MiCOM P546 - 2.6.1 Directional overcurrent characteristic angle settings; 2.7 Thermal overload protection; 2.7.1 Single time constant characteristic; 2.7.2 Dual time constant characteristic

Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-37
AP
2.6.1 Directional overcurrent characteristic angle settings
The relay uses a 90° connection angle for the directional overcurrent elements. The relay
characteristic angles in this case are nominally set to:
 +30° Plain feeders, zero sequence source behind relay
 +45° Transformer feeder, zero sequence source in front of relay
Whilst it is possible to set the RCA to exactly match the system fault angle, it is
recommended that the above figures are followed, as these settings have been shown to
provide satisfactory performance and stability under a wide range of system conditions.
2.7 Thermal overload protection
Therm
al overload protection can be used to prevent electrical plant from operating at
temperatures in excess of the designed maximum withstand. Prolonged overloading causes
excessive heating, which may result in premature ageing of the insulation, or in extreme
cases, insulation failure.
2.7.1 Single time constant characteristic
The cu
rrent setting is calculated as:
Thermal Trip = Permissible continuous loading of the plant item/CT ratio.
Typical time constant values are given in the following table. The relay setting, ‘Time
Constant 1’, is in minutes.
Time constant 
(minutes)
Limits
Air-core reactors 40
Capacitor banks 10
Overhead lines 10
Cross section  100 mm2 Cu or 150 mm2 Al
Cables 60 - 90 Typical, at 66 kV and above
Busbars 60
An alarm can be raised on reaching a thermal state corresponding to a percentage of the trip
threshold. A typical setting might be ‘Thermal Alarm’ = 70% of thermal capacity.
2.7.2 Dual time constant characteristic
The cu
rrent setting is calculated as:
Thermal Trip = Permissible continuous loading of the transformer / CT ratio.
Typical time constants:
1 (minutes) 2 (minutes)
Limits
Oil-filled transformer 5 120 Rating 400 - 1600 kVA
An alarm can be raised on reaching a thermal state corresponding to a percentage of the trip
threshold. A typical setting might be ‘Thermal Alarm’ = 70% of thermal capacity.
Note: The thermal time constants given in the above tables are typical only.
Reference should always be made to the plant manufacturer for
accurate information.